Effective Objective-C 2.0: 英文电子书指南

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"ios mac Effective Objective-C 2.0 开发学习文档，英文版本" 这篇学习文档专注于Objective-C 2.0这一编程语言在iOS和Mac平台上的应用，它旨在帮助开发者提升开发效率和代码质量。Objective-C是Apple的C语言扩展，主要用于构建iOS和macOS的应用程序。该文档是英文版，因此对英文阅读能力有一定的要求，但同时也能提供最直接、最准确的学习体验。 Objective-C 2.0主要增加了许多新特性，如块（Blocks）、分类（Categories）和垃圾回收（Garbage Collection），这些都极大地提升了开发者的生产力。以下是关于这些关键知识点的详细说明： 1. **块（Blocks）**：块是Objective-C 2.0引入的一种闭包实现方式，允许将代码段作为一个对象来处理。这在处理异步任务、回调函数和算法定义时非常有用。块可以捕获并存储其定义时的作用域内的变量，使得代码更加简洁和灵活。 2. **分类（Categories）**：分类允许开发者向已有的Objective-C类添加方法，而无需继承或修改原有类的源码。这在不改变原有类结构的情况下扩展其功能非常有用，比如在Foundation框架中，`NSArray`和`NSMutableArray`就通过分类添加了许多实用方法。 3. **垃圾回收（Garbage Collection）**：在Objective-C 2.0中，垃圾回收机制被引入以自动管理内存。然而，后来的iOS版本中，Apple更倾向于推荐使用Automatic Reference Counting (ARC)，因为ARC在性能和内存管理上更为高效。垃圾回收在某些情况下可能引发延迟，因此理解ARC的工作原理对于现代iOS开发至关重要。 4. **ePUB格式**：这份文档采用ePUB格式，是一种开放的电子书标准，允许内容在不同设备和应用程序上适应性展示。读者可以根据自己的偏好调整字体、字号、列布局以及查看模式（横竖屏）。对于包含代码或配置示例的部分，文档可能会提供可点击或触摸放大图，以保持代码的清晰度。 5. **编程代码和配置示例**：为了优化代码和配置的显示，建议在阅读设备上选择单栏、横屏模式，并减小字体大小。部分代码和配置会以可流动文本格式呈现，但为了与印刷版保持一致，还提供了图像链接。点击链接可以看到与印刷版相似的代码图片，如果需要返回前一页，可以使用设备或应用的后退按钮。这份文档深入讲解了Objective-C 2.0的关键概念和技术，适合有一定Objective-C基础并且希望深化理解或更新知识的开发者阅读。通过阅读和实践文档中的内容，开发者不仅可以掌握Objective-C 2.0的新特性，还能了解到如何在实际项目中有效应用这些技术，提升iOS和macOS应用的开发水平。

1. Accustoming Yourself to Objective-C

Objective-C brings object-oriented features to C through an entirely new syntax.

Often described as verbose, Objective-C syntax makes use of a lot of square brackets

and isn’t shy about using extremely long method names. The resulting source code is

very readable but is often difficult for C++ or Java developers to master.

Writing Objective-C can be learned quickly but has many intricacies to be aware of

and features that are often overlooked. Similarly, some features are abused or not fully

understood, yielding code that is difficult to maintain or to debug. This chapter covers

fundamental topics; subsequent chapters cover specific areas of the language and

associated frameworks.

Item 1: Familiarize Yourself with Objective-C’s Roots

Objective-C is similar to other object-oriented languages, such as C++ and Java, but

also differs in many ways. If you have experience in another object-oriented language,

you’ll understand many of the paradigms and patterns used. However, the syntax may

appear alien because it uses a messaging structure rather than function calling.

Objective-C evolved from Smalltalk, the origin of messaging. The difference between

messaging and function calling looks like this:

// Messaging (Objective-C)

Object *obj = [Object new];

[obj performWith:parameter1 and:parameter2];

// Function calling (C++)

Object *obj = new Object;

obj->perform(parameter1, parameter2);

The key difference is that in the messaging structure, the runtime decides which

code gets executed. With function calling, the compiler decides which code will be

executed. When polymorphism is introduced to the function-calling example, a form of

runtime lookup is involved through what is known as a virtual table. But with messaging,

the lookup is always at runtime. In fact, the compiler doesn’t even care about the type of

the object being messaged. That is looked up at runtime as well, through a process

known as dynamic binding, covered in more detail in Item 11.

The Objective-C runtime component, rather than the compiler, does most of the

heavy lifting. The runtime contains all the data structures and functions that are

required for the object-oriented features of Objective-C to work. For example, the

runtime includes all the memory-management methods. Essentially, the runtime is the

set of code that glues together all your code and comes in the form of a dynamic library

to which your code is linked. Thus, whenever the runtime is updated, your application

benefits from the performance improvements. A language that does more work at

compile time needs to be recompiled to benefit from such performance improvements.

Objective-C is a superset of C, so all the features in the C language are available

when writing Objective-C. Therefore, to write effective Objective-C, you need to

understand the core concepts of both C and Objective-C. In particular, understanding

the memory model of C will help you to understand the memory model of Objective-C

and why reference counting works the way it does. This involves understanding that a

pointer is used to denote an object in Objective-C. When you declare a variable that is

to hold a reference to an object, the syntax looks like this:

NSString *someString = @"The string";

This syntax, mostly lifted straight from C, declares a variable called someString whose

type is NSString*. This means that it is a pointer to an NSString. All Objective-C objects

must be declared in this way because the memory for objects is always allocated in

heap space and never on the stack. It is illegal to declare a stack-allocated Objective-C

object:

NSString stackString;

// error: interface type cannot be statically allocated

The someString variable points to some memory, allocated in the heap, containing an

NSString object. This means that creating another variable pointing to the same location

does not create a copy but rather yields two variables pointing to the same object:

NSString *someString = @"The string";

NSString *anotherString = someString;

There is only one NSString instance here, but two variables are pointing to the same

instance. These two variables are of type NSString*, meaning that the current stack

frame has allocated 2 bits of memory the size of a pointer (4 bytes for a 32-bit

architecture, 8 bytes for a 64-bit architecture). These bits of memory will contain the

same value: the memory address of the NSString instance.

Figure 1.1 illustrates this layout. The data stored for the NSString instance includes

the bytes needed to represent the actual string.

heap memory. When nonobject types (int, float, double, char, etc.) are the only data to

be held, a structure, such as CGRect, is usually used.

Before embarking on writing anything in Objective-C, I encourage you to read texts

about the C language and become familiar with the syntax. If you dive straight into

Objective-C, you may find certain parts of the syntax confusing.

Things to Remember

Objective-C is a superset of C, adding object-oriented features. Objective-C uses a

messaging structure with dynamic binding, meaning that the type of an object is

discovered at runtime. The runtime, rather than the compiler, works out what code to

run for a given message.

Understanding the core concepts of C will help you write effective Objective-C. In

particular, you need to understand the memory model and pointers.

Item 2: Minimize Importing Headers in Headers

Objective-C, just like C and C++, makes use of header files and implementation files.

When a class is written in Objective-C, the standard approach is to create one of each

of these files named after the class, suffixed with .h for the header file and .m for the

implementation file. When you create a class, it might end up looking like this:

// EOCPerson.h

#import <Foundation/Foundation.h>

@interface EOCPerson : NSObject

@property (nonatomic, copy) NSString *firstName;

@property (nonatomic, copy) NSString *lastName;

@end

// EOCPerson.m

#import "EOCPerson.h"

@implementation EOCPerson

// Implementation of methods

@end

The importing of Foundation.h is required pretty much for all classes you will ever

make in Objective-C. Either that, or you will import the base header file for the

framework in which the class’s superclass lives. For example, if you were creating an

iOS application, you would subclass UIViewController often. These classes’ header

files will import UIKit.h.

As it stands, this class is fine. It imports the entirety of Foundation, but that doesn’t

matter. Given that this class inherits from a class that’s part of Foundation, it’s likely that

a large proportion of it will be used by consumers of EOCPerson. The same goes for a

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